What is socialisation?
Socialisation in puppies is when the puppy learns to get along with other pups, people and other species of animal which they are likely to come in contact with later in life.
They don't neccessarily need to be best friends with these animals. A socialised pup might learn how to avoid getting scratched by a cat by leaving it alone and giving it space for example.
This video is the ultimate in socialisation between 2 different species: